1. Field of the Invention
The present invention relates to a planar light wave circuit, and more particularly to a planar lightwave circuit having an optical filter.
2. Description of the Related Art
Recently, communication networks of FTTH (Fiber-To-The-Home) types and FTTx (x: Premises, Home, Business, etc.) in which a central base station is linked to subscribers through optical fibers are used. The optical communication method for these FTTH or the FTTx networks require various types of optical transmitting and receiving devices for performing bi-directional communications.
The optical transmitting and receiving devices used for bi-directional communications typically include detecting data after converting optical signals of the corresponding waves to electrical signals and then transmitting the data to the receiving side after the conversion of electrical data into optical signals. An integrated planar lightwave circuit is mainly used as the optical transmitting and receiving device. More specifically, planar lightwave circuits having wave division multiplexing or demultiplexing filters mounted thereto are being used. Recently, planar lightwave circuits equipped with optical filters having various functions have been deployed for various applications in the optical communication network.
FIG. 1 shows a planar lightwave circuit having an optical filter. Referring to FIG. 1, the planar lightwave device 100 comprises first and second waveguides 111 and 112 separated from each other, a trench 120 which separates the first waveguide 111 from the second waveguide 112 by a predetermined distance, and an optical filter 130 positioned in the trench 120. The planar lightwave circuit 100 forms the first and second waveguides 111 and 112 using a lower clad, a core, and an upper clad that are sequentially stacked on the semiconductor substrate.
The trench 120 is formed by dicing the lower clad, the core, and the upper clad, and as a result, it separates the first waveguide 111 from the second waveguide 112 by a predetermined distance. The optical filter 130 is positioned in the trench 120 and between the first and second waveguides 111 and 112, and a wave division demultiplexing filter for demultiplexing the inputted optical signals is used. However, since the waveguides are separated from each other by the trench, the proceeding optical signals are diffused in the trench and as a result, some of the signals fail to converge toward the other side waveguide, thereby causing insertion losses of the optical signals.
To address the above problem, a planar lightwave circuit using a multi-mode waveguide has been suggested as a means for restraining the generation of insertion losses. However, the multimode waveguide does not provide a sufficient coupling efficiency suitable for a real product.